Three-core high temperature superconducting cables (three-core HTS cables) are known as conventional alternating current (AC) superconducting cables (see, for example, Japanese Patent Application Publication Nos. 9-134620 and 2001-202837). In these cables, a three-core stranded unit is housed in a thermal insulation pipe. Each cable core is formed of, sequentially from the center, a former, a superconducting cable conductor, an electrical insulating layer, and a shielding layer. Generally, a space formed in the former and a space formed between the thermal insulation pipe and each cable core serve as coolant channels. The superconducting cable conductor is formed by stranding superconducting wires in multilayers, and the spiral pitch and the winding direction are adjusted so that the impedances of the layers become uniform to allow a current to flow uniformly. The shielding layer is formed by using superconducting wires in a similar structure as the superconducting cable conductor.
As described above, in AC superconducting cables, a conductor is formed by winding the superconducting wires spirally so that the current may become radially uniform. However, AC loss is caused by the inductance, and the transmission current is restricted to that extent. Also, in the AC line system, the short-circuit current becomes large in the event of failure, and as a result, the temperature increases, which is also disadvantageous.
Accordingly, transmission using DC superconducting cables is also considered. DC superconducting cables are used for long-distance or bulk-power transmission. The conductor size must be increased in order to perform bulk-power transmission, and thus, the use of single-core cables has been considered.
However, known single-core DC superconducting cables have the following drawbacks.
(1) It is difficult to simplify the cooling system.
For conducting bipolar power transmission, a plurality of cables is required. However, since the superconducting cables are independently cooled in practice, the system becomes complicated, and the system cost also increases.
(2) It is difficult to take measures against the contraction of cable cores occurring during cooling.
In known AC cables, in order to provide an allowance for thermal contraction of cores, a three-core stranded unit is loosened in advance, or a spacer is interposed at the center of the stranded cores. However, in DC superconducting cables using single-core cables, measures against contraction cannot be taken by the adjustment of stranding. If the cores are not loosened, a tensile stress is generated in superconducting cable wires during cooling. This requires the use of wires resistant to this tensile stress. Thus, it is difficult to implement measures against the contraction.